This invention relates to synthetic aperture radar (SAR), and more particularly to a method and apparatus for using a single-frequency multibeam antenna for imaging separate range strips of a large swath at high range intervals.
In some applications for SAR, it is desirable to image a swath much greater than is possible for a given interpulse interval. For example, in global oceanography, SAR may be used to monitor ocean states. The ocean wave spectra does not change very rapidly spatially, and so samples of the wave spectra extending over a large area are necessary to enable modeling and prediction of ocean conditions. When the SAR sends out a succession of pulses, the time delay for the pulse return provides the range resolution and Doppler processing of the returns for a series of pulses provides the azimuth resolution for the images.
While the time between successive pulses determines the maximum range that can be mapped by the radar system, too low pulse repetition rate results in azimuth ambiguities due to Doppler foldover of the matched filtered image output. The unambiguous range that can be mapped is given by cD/8v.sub.a, where c is the velocity of light, D the antenna dimension and v.sub.a the velocity of the radar platform. This is insuffiunambiguous range for oceanography which requires monitoring ocean conditions over swaths of 10-km widths centered at 100-km intervals up to 1500 km of range. This requires a multibeam system, each beam illuminating a different range swath. The problem is achieving this with a single frequency radar system.